For some time, known tank-type flush toilets flushed by stored flush water to discharge waste have included those in which flush water stored in a reservoir tank passes through a water passageway in a toilet main unit to be supplied to a bowl portion from a spout port in the toilet main unit. In such flush toilets, the water passageway in the toilet main unit is formed of porcelain, and because pressure losses in the water passageway fluctuate due to manufacturing errors, etc. in the porcelain water passageway, there are also fluctuations in the instantaneous flow rate, etc. of flush water discharged from a discharge port on the bottom surface of a reservoir tank as the result of these pressure losses. In particular, those toilets in which a discharge valve which opens and closes the discharge port inside the reservoir tank is opened for a fixed time under timer control are directly affected by pressure losses in the water passageway, etc., leading to variability in the amount of flush water discharged from the reservoir tank to the toilet main unit.
Known solutions to such problems include, for example, the one set forth in Patent Document 1 (Japanese Patent Unexamined Publication No. 2012-132167), including a flush control device in which the time is measured for a drop in the water level inside a reservoir tank, from an initial water level prior to start of flush to a predetermined water level, and the spouted flow rate of water spouted from the spout port into the bowl portion of the toilet main unit is adjusted based on this measured water level drop time.